Coeval boudinage and folding of oblique single layers under coaxial plane strain: Layer rotation around the principal stretching axis (X)

Abstract

We carried out scaled analogue modelling to show the influence of varying layer obliquity on the geometry of single-layer folds and boudins, which developed simultaneously under coaxial plane strain. The initial angle (θZ(i)) between the competent layer and the shortening axis (Z) was gradually changed by turning the layer around the principal stretching axis (X). Although oblique-layer rotation was similar fast like that of a corresponding passive plane, each layer was crossing different strain fields (shortening, reduced shortening, elongation) resulting in coeval boudinage and F1-folding, the latter with fold axes parallel to X. A rise in θZ(i) led to increase of boudinage and decrease of F1-folding. The amplitude of F1-folds increased with viscosity ratio between layer and matrix but was hardly affected by θZ(i). The arc- and wavelength of F1-folds, on the other hand, increased if θZ(i) > 45°. In cases of low viscosity ratio between layer and matrix, boudins were affected by F2-folds with axes subperpendicular to the layer. Both F1- and F2-folds show increasing interlimb angles at rising θZ(i).The structures of the present study are largely not cylindrical and require investigation in three dimensions. They are frequent in high-pressure/low-temperature metamorphic rocks of subduction-zone settings and in salt walls.